1. Field of the Invention
The present invention generally relates to systems for controlling sheetmaking machines and, more particularly, to control systems that employ measuring devices that obtain cross-directional profiles across sheets during production.
2. State of the Art
It is common practice to make on-line measurements of sheet properties during sheet processing. The purpose of on-line measurements is to enable prompt control of sheet processing and, thus, to increase sheet uniformity and to reduce the quantity of substandard sheet material that is produced before undesirable process conditions are corrected. In papermaking, for instance, on-line sensors detect properties such as basis weight, moisture content, and caliper of paper sheets.
It is well known that on-line measurements can be made by sensors that periodically traverse, or "scan", traveling sheets in the cross direction, i.e., in a direction extending across the sheet surface generally perpendicular to the direction of sheet travel. In practice, scanning sensors typically travel at a rate of about ten inches per second and provide an integrated output about every fifty milliseconds; thus, a typical scanning sensor provides about one measurement output per each half-inch in the cross direction. Measurement information detected by scanning sensors is often assembled to provide graph-like "profiles" composed of successions of measurements at adjacent locations in the cross direction, which locations are usually are referred to as "slices". Equivalent cross-directional profiles also can be obtained by measurement systems that employ arrays of fixed sensors that extend in the cross direction.
To control sheetprocessing machines to produce uniform quality sheets, it is common practice to detect variations that occur both between and within cross-directional profiles. In the machine direction, sheet uniformity can be defined as a measured sheet property being constant from profile to profile along any given slice. In the cross direction, sheet uniformity can be defined as a measured sheet property being constant from slice to slice; thus, cross-directional sheet uniformity implies flat profiles.
Various devices are known to control sheet properties in the cross direction and, hence, to control cross-directional profiles. In papermaking, for instance, basis weight can be controlled in the cross direction by a headbox, either of the primary or secondary type. As another example, variations in moisture across a paper sheet can be controlled by a re-moisturizing shower which has independently controllable segments in the cross direction.
A difficulty in using profile measurements for cross-directional control of sheet properties arises when a sheetprocessing machine employs two or more profiling devices that are both capable of affecting the same property of a sheet. During the manufacture of paper, for instance, sheet moisture can be adjusted in the cross direction by both steamboxes and remoisturizing showers and, often, papermaking machines include both devices. As another example from the papermaking field, basis weight can be adjusted in the cross direction by both primary and secondary headboxes, and some papermaking machines include both devices. In such situations, a question arises as to how the two devices should be controlled when the control signals for both devices, at any given time, are derived from the same cross-directional profile.
In conventional control practice, multiple control devices on sheetprocessing machines are often operated with actuating signals distributed, at any given time, according to a predetermined ratio. As one example, where both a re-moisturizing shower and a steambox are employed on the same papermaking machine, a conventional control strategy might be that, for a given magnitude of the actuating signal at a given time, the re-moisturizing shower is turned on one-fourth and the steam box is turned on one-eighth. Then, according to this example, when the actuating signal doubles in magnitude, the remoisturizing shower is turned on one-half and the steam box one-quarter. Such practice is often referred to as "ratioing" of signals.
Another conventional strategy for operating two or more control devices on a sheetprocessing machine is called "mid-ranging". In a typical mid-ranging strategy, a first control device is operated as long as the set points of its actuators fall within predetermined limits, i.e., within a defined "mid-range". Then, when one or more of the set points moves outside the mid-range, a second control device is actuated. Normally, limits for the mid-ranges are selected based upon prior operating experience and by the capabilities of the control device actuators
It should be appreciated that conventional control strategies, such as ratioing and mid-ranging techniques, do not necessarily result in optimum results when applied to control devices on sheetprocessing machines. In other words, conventional control strategies do not necessarily achieve optimum use of the control device which is best suited to adjust process conditions to overcome detected non-uniformities in sheet profiles.